STUDY ON STRENGTH OF CONCRETE USING VARIOUS COARSE AGGREGATES, PART-1

In the modern era, concrete is one of the most important materials of construction. Countries like Bangladesh where steel is not easily available and costly, concrete structures become economical.

Concrete is an artificial stone like material obtained by permitting a carefully proportioned mixture of cement, coarse aggregate, fine aggregate and water to harden in forms of the shape and dimensions of the desired structure. Concrete in a wide range of strength properties can be obtained by appropriate adjustment of the proportions of the constituent materials.

1.1.1 Composition of concrete

Concrete is a composite material which consists essentially of a binding medium within which are embedded particles or fragments of relatively inert mineral filler. In cement concrete the binder or matrix, either in the plastic or in the hardened state, is a combination of cement and water; it is commonly called the “cement paste”. The filler material, called “aggregate”, is generally graded in size from a fine sand to pebbles or fragments of stone which, in some concretes, may be several inches in diameter.

The aggregate occupies roughly three- quarters of the space within a given mass. The space not occupied by aggregates, roughly one quarter of the entire volume of an average concrete, is filled with cement paste and air voids. Cement and water interact chemically to bind the aggregate particles into a solid mass.

1.1.2 Properties of concrete

The concrete possesses the following important properties:

(a) It has a high compressive strength.

(b) It is free from corrosion and there is no appreciable effect of atmospheric agents on it.

(c) It hardens with age and the process of hardening continues for a long time after

Crushed Stone as Coarse Aggregate

the concrete has attained sufficient strength.

Coarse Sand

(d) It is proved to be more economical than steel.

(e) It binds rapidly with steel and as it is weak in tension, the steel reinforcement is placed in cement concrete at suitable places to take up the tensile stresses.

(f) It forms a hard surface, capable of resisting abrasion.

1.2 Aggregate

Aggregates (both coarse as well as fine) are mineral materials like sand, gravel, crushed stone etc. which do not undergo any chemical change, but simply provide mass or volume to the concrete and reduce shrinkage effects in it.

1.2.1 Functions of aggregate

Fine Sand

The aggregate has three principal functions:

1. To provide a relatively cheap filler for the cementing material.

2. To provide a mass of particles which are suitable for resisting the action of applied loads, abrasion, the percolation of moisture and action of weather.

3. To reduce the volume changes resulting from the setting and hardening process and from moisture changes in the cement-water paste.